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SubjectRe: [PATCH v2] tomoyo: Don't check open/getattr permission on sockets.
Hi Tetsuo,

On Sat, Jun 22, 2019 at 01:45:30PM +0900, Tetsuo Handa wrote:
> On 2019/06/19 5:49, Al Viro wrote:
> > On Sun, Jun 16, 2019 at 03:49:00PM +0900, Tetsuo Handa wrote:
> >> Hello, Al.
> >>
> >> Q1: Do you agree that we should fix TOMOYO side rather than SOCKET_I()->sk
> >> management.
> >
> > You do realize that sockets are not unique in that respect, right?
> > All kinds of interesting stuff can be accessed via /proc/*/fd/*, and
> > it _can_ be closed under you. So I'd suggest checking how your code
> > copes with similar for pipes, FIFOs, epoll, etc., accessed that way...
>
> I know all kinds of interesting stuff can be accessed via /proc/*/fd/*,
> and it _can_ be closed under me.
>
> Regarding sockets, I was accessing "struct socket" memory and
> "struct sock" memory which are outside of "struct inode" memory.
>
> But regarding other objects, I am accessing "struct dentry" memory,
> "struct super_block" memory and "struct inode" memory. I'm expecting
> that these memory can't be kfree()d as long as "struct path" holds
> a reference. Is my expectation correct?
>
> >
> > We are _not_ going to be checking that in fs/open.c - the stuff found
> > via /proc/*/fd/* can have the associated file closed by the time
> > we get to calling ->open() and we won't know that until said call.
>
> OK. Then, fixing TOMOYO side is the correct way.
>
> >
> >> Q2: Do you see any problem with using f->f_path.dentry->d_inode ?
> >> Do we need to use d_backing_inode() or d_inode() ?
> >
> > Huh? What's wrong with file_inode(f), in the first place? And
> > just when can that be NULL, while we are at it?
>
> Oh, I was not aware of file_inode(). Thanks.
>
> >
> >>> static int tomoyo_inode_getattr(const struct path *path)
> >>> {
> >>> + /* It is not safe to call tomoyo_get_socket_name(). */
> >>> + if (path->dentry->d_inode && S_ISSOCK(path->dentry->d_inode->i_mode))
> >>> + return 0;
> >
> > Can that be called for a negative?
> >
>
> I check for NULL when I'm not sure it is guaranteed to hold a valid pointer.
> You meant "we are sure that path->dentry->d_inode is valid", don't you?
>
> By the way, "negative" associates with IS_ERR() range. I guess that
> "NULL" is the better name...
>
> Anyway, here is V2 patch.
>
> From c63c4074300921d6d1c33c3b8dc9c84ebfededf5 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Date: Sat, 22 Jun 2019 13:14:26 +0900
> Subject: [PATCH v2] tomoyo: Don't check open/getattr permission on sockets.
>
> syzbot is reporting that use of SOCKET_I()->sk from open() can result in
> use after free problem [1], for socket's inode is still reachable via
> /proc/pid/fd/n despite destruction of SOCKET_I()->sk already completed.
>
> But there is no point with calling security_file_open() on sockets
> because open("/proc/pid/fd/n", !O_PATH) on sockets fails with -ENXIO.
>
> There is some point with calling security_inode_getattr() on sockets
> because stat("/proc/pid/fd/n") and fstat(open("/proc/pid/fd/n", O_PATH))
> are valid. If we want to access "struct sock"->sk_{family,type,protocol}
> fields, we will need to use security_socket_post_create() hook and
> security_inode_free() hook in order to remember these fields because
> security_sk_free() hook is called before the inode is destructed. But
> since information which can be protected by checking
> security_inode_getattr() on sockets is trivial, let's not be bothered by
> "struct inode"->i_security management.
>
> There is point with calling security_file_ioctl() on sockets. Since
> ioctl(open("/proc/pid/fd/n", O_PATH)) is invalid, security_file_ioctl()
> on sockets should remain safe.
>
> [1] https://syzkaller.appspot.com/bug?id=73d590010454403d55164cca23bd0565b1eb3b74
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Reported-by: syzbot <syzbot+0341f6a4d729d4e0acf1@syzkaller.appspotmail.com>
> ---
> security/tomoyo/tomoyo.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
> index 716c92e..8ea3f5d 100644
> --- a/security/tomoyo/tomoyo.c
> +++ b/security/tomoyo/tomoyo.c
> @@ -126,6 +126,9 @@ static int tomoyo_bprm_check_security(struct linux_binprm *bprm)
> */
> static int tomoyo_inode_getattr(const struct path *path)
> {
> + /* It is not safe to call tomoyo_get_socket_name(). */
> + if (S_ISSOCK(d_inode(path->dentry)->i_mode))
> + return 0;
> return tomoyo_path_perm(TOMOYO_TYPE_GETATTR, path, NULL);
> }
>
> @@ -316,6 +319,9 @@ static int tomoyo_file_open(struct file *f)
> /* Don't check read permission here if called from do_execve(). */
> if (current->in_execve)
> return 0;
> + /* Sockets can't be opened by open(). */
> + if (S_ISSOCK(file_inode(f)->i_mode))
> + return 0;
> return tomoyo_check_open_permission(tomoyo_domain(), &f->f_path,
> f->f_flags);
> }
> --

What happened to this patch?

Also, isn't the same bug in other places too?:

- tomoyo_path_chmod()
- tomoyo_path_chown()
- smack_inode_getsecurity()
- smack_inode_setsecurity()

- Eric

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